If you override the getKey method of the Ext.util.MixedCollection data object within an Ext.data.Store, the grid view is no longer able to link records selected in the view to records in the Store. The issue stems from Ext.view.AbstractView. In the updateIndexes private method, a custom attribute called viewRecordId is set to the record's internalId property:

This isn't really a bug. In this instance you're messing around with the internals of the class. Obviously this is "allowed", however by the same token you need to account for such things in your own code. The store itself explicitly passes a getKey function already, which you're overriding.